This invention relates to the field of fabricating articles from a plurality of plies of fiber reinforced resin matrix composite material. More particularly, the invention relates to a mandreless molding system to produce components which have molded outside surfaces with intermittent molded internal surfaces such as an aircraft skin with internal stiffeners typically known as longerons, stringers, frames, and struts.
The use of fiber reinforced resin matrix composite materials in the fabrication of aircraft components is becoming increasingly more popular and therefore more important. For instance the F-15 fighter, currently the premier fighter of the U.S. Air Force, developed in the early 1970's had only four percent of its structural weight made up of composites. Comparatively, the advanced tactical fighter, currently in competition, has approximately fifty percent; the B-2 bomber is estimated to have 80 percent of the B-2 structure made from composites.
Composites, as applied to this invention, are made of two essential ingredients: high-strengthened fibers that have been woven into tape or broadcloth and resins that bind the fabric into a rigid and light-weight matrix. Carbon, graphite, and glass are the most typical fibers used. Epoxy, polyester, and phenolics are the most common polymers used. Typically, in producing parts, the tape or broadcloth, which is impregnated with the polymer and known as a prepreg, is laid down onto a mold that represents the shape and contour of the outer skin of the aircraft or helicopter. These stiffeners, longerons and stringers, are typically made from prepreg material preformed into an efficient structural cross section, e.g. T, Z, C, and hat sections which all have a substantial amount of open space. These stiffeners are then arranged on the above-noted skin along dominate load directions both longitudinally and transversely. Since these stiffener members are required to strain with the skin or shell member in order to carry a portion of the load they must be bonded to the skin. This is usually accomplished by co-curing the stiffener along with the skin. However, mandrels must be placed inside these structural stiffeners in order to shape and prevent their collapse during curing in an autoclave at a temperature of about 350.degree. and a pressure of 100 pounds per square inch.
By aligning the stiffeners along dominant load directions a more efficient structural arrangement is achieved. For this reason, it is common practice to run the stiffeners in a fuselage generally along the length of the structure. As previously indicated, the stiffeners themselves must be supported at intervals by transverse stiffening members to prevent their failure by column buckling. This intersection of the longitudinal and transverse stiffeners requires an integral joint even though the longerons generally run continuously and are butted by the transverse members. Structural requirements of these intersecting stiffener joints make for very difficult and sometimes impossible removal of the above-noted mandrels which are trapped. Sometimes it is necessary to use "flyaway" mandrels because they cannot be removed.
In today's typical art, the above-noted prepreg skin with the prepreg stiffeners in place, all supported by the mold and mandrels, as required, is covered by a vacuum bag and a vacuum drawn between the bag and the mold to compress the parts before they are placed in the autoclave. These are generally throw away vacuum bags. Throw away vacuum bags are both costly and labor intensive as it takes a long time to bag a structural assembly.
It is an object of the present invention to produce a tool which will allow for the fabrication of fiber reinforced resin matrix composite aircraft and helicopter structures without requiring the use of mandrels. It is a further object of this invention to integrally mold mating surfaces on the inside as well as the outside surface of the component. It is yet another object of the invention to reduce the cost of the tooling by elimination of mandrels and disposable bagging materials and, further, substantially reduce the manpower required to both vacuum bag and remove the mandrels from a structural part. The final object of the invention is to be able to use several different cross section stiffeners without changing the tooling.